The growth of the microelectronics industry has in large part been made possible by rapid advances in microfabrication technologies. Through miniaturization of microelectronic structures, the number of devices which can be placed on an integrated circuit has increased exponentially for several decades. This increase in packing density has led to the development of smaller, faster and more affordable microelectronic devices. Due to a number of limitations of conventional fabrication techniques, however, device miniaturization faces a number of obstacles. For example, as circuit designers further reduce the size of various structures, traditional photolithography techniques are limited by the wavelength of the light and the manufacture of masks which are used for patterning. Accordingly, alternative techniques have been explored for fabrication of microelectronics as well as other thin-film devices such as microelectrodes and microsensors and the like.
While microtechnology is relatively mature, development of nanotechniques attracts more and more attention. In nanotechnology, nanometer or molecular-scale structures are fabricated. "Nanoetching" generally includes those techniques which are used to remove material from a substrate to create a nanoscale structure, for example, a nanometer-scale groove or trench. "Molecular nanotechnology" often refers to the fabrication of nanometer-scale, three dimensional structures by the assembly of individual molecules. "Nanografting" has been used to describe the deposition of a material with nanometer dimensions to a substrate. Regardless of the nomenclature, it is clear that for device miniaturization to reach its full potential, fabrication methods must satisfy the resolution requirements of industry and must become a commercial reality.
In "Nanografting of N-Vinyl-2-Pyrrolidone Molecules on a Graphite Surface with a Scanning Tunneling Microscope", Langmuir, Vol. 12, 3252-3256, Chen, et al. disclose the use of a scanning tunneling microscopy tip to induce grafting of NVP molecules on a graphite surface from a solution of NVP. At high bias voltages, pits or holes were formed in the surface of the graphite substrate and islands of NVP (or its derivatives) were deposited in the pits from solution. The islands were reportedly of nanometer scale.
A number of organic molecules are known to self-assemble spontaneously on certain substrates. These self-assembled monolayers (SAMs) are essentially oriented monolayers with head groups chemically attached to the substrate and end groups exposed at the interface. In "Adsorption of Bifunctional Organic Disulfides on Gold Surfaces, J. Am. Chem. Soc., Vol. 105, No. 13, 1983, the entire disclosure of which is incorporated herein by reference, Nuzzo, et al. disclose the preparation and structural characterization of supported monolayer assemblies of oriented organic molecules by adsorption of thiols and disulfides on gold substrates. The self assembly of long-chain alkanethiols was further investigated by Bain et al. as disclosed in "Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols form Solution onto Gold", J. Am. Chem. Soc. Vol. 111, 321-335 (1989), the entire disclosure of which is incorporated herein by reference. Therein, it was disclosed that long-chain alkanethiols, HS(CH.sub.2).sub.n X, adsorb from solution onto gold surfaces and form ordered, oriented monolayers.
It is also known that these self-assembled monolayers can be patterned using electron beams, metastable argon beams, photolithography and scanning probe microscopy. In addition, two or more self-assembled monolayers on gold have been patterned with a resolution of from 0.1 micron to 100 microns by microcontact printing, micromachining and microwriting. In "The Use of Self-Assembled Monolayers and a Selective Etch to Generate Patterned Gold Features", J. Am. Chem. Soc. Vol. 114, 9188-9189 (1992), Kumar et al. disclose the use of self-assembled monolayers as a resist on gold film to block etching using a solution KOH and KCN. In addition, a monolayer SAM on a gold film substrate was micromachined and etched to form a trench in the gold layer. In "Using Micromachining, Molecular Self-Assembly, and Wet Etching to Fabricate 0.1-1 micron Scale Structures of Gold and Silicon", Chem Mater, Vol. 6, No. 5 (1994), Kumar et al. further describe this machining technique. Therein, a monolayer of HO(CH.sub.2).sub.2 S was chemisorbed on the surface of a gold film on a glass substrate. Grooves of 0.1-1.0 micron were then machined in the adsorbed layer with the tip of a surgical blade or the cut end of a carbon fiber. Bare gold was exposed in the grooves, Kumar et al. disclose that the dimensions of the micromachined regions of bare gold were influenced by both the shape of the machining tip and the size of the mechanical load applied to the tip. The third step of patterning was the immersion in a SAM of CH.sub.3 (CH.sub.2).sub.15 S. It was reported that the SAM molecules self-assembled on the exposed gold. The areas of gold film covered by the HO(CH.sub.2).sub.2 S were then selectively etched to remove the HO(CH.sub.2).sub.2 S and the underlying gold. Kumar et al. state that the procedure can be scaled to dimensions of less than 10 nanometers by using scanning tunneling microscopy and atomic force microscopy to micromachine SAMS formed on the surface of metal films.
In "Reversible Displacement of Chemisorbed n-Alkanethiol Molecules on AU(111) Surface: An Atomic Force Microscopy", Langmuir, Vol. 10, 367-370 (1994), the entire disclosure of which is incorporated by reference, one of the present inventors discloses that Atomic Force Microscopy (AFM) can be used to displace a preselected region of a SAM on a gold substrate and that after removal of the applied load the displaced region is restored. This reversible displacement was demonstrated independently with SAMs formed from CH.sub.3 (CH.sub.2).sub.9 SH and from CH.sub.3 (CH.sub.2).sub.17 SH.
Thus, there exists a need for a method of forming higher resolution, e.g. nanometer or molecular scale, patterns within the plane of an adsorbate on a substrate.